The present invention relates to a sintered carbonitride alloy with titanium as main component and containing molybdenum. This alloy is preferably used as an insert for milling and turning. By starting the sintering with an oxidizing treatment, it is possible to obtain a high molybdenum-content in the binder phase which gives the alloy improved properties.
Classic cemented carbide, i.e., based upon tungsten carbide (WC) and with cobalt (Co) as binder phase has in the last few years met with increased competition from titanium-based hard materials, usually called cermets. In the beginning, these titanium-based alloys were used only for high speed finishing because of their extraordinary wear resistance at high cutting temperatures. This property depends essentially upon the good chemical stability of these titanium-based alloys. The toughness behavior and resistance to plastic deformation were not satisfactory, however, and therefore the area of application was relatively limited.
Development has proceeded and the area of application for sintered titanium-based hard materials has been considerably enlarged. The toughness behavior and the resistance to plastic deformation have been considerably improved. This has been done, however, by partly sacrificing the wear resistance.
An important development in titanium based hard alloys is the substitution of carbides by nitrides in the hard constituent phase. This decreases the grain size of the hard constituents in the sintered alloy. Both the decrease in grain size and the use of nitrides lead to the possibility of increasing the toughness at unchanged wear resistance. Characteristic for said alloys is that they are usually considerably more finegrained than normal cemented carbide, i.e., WC-Co-based hard alloy. Nitrides are also generally more chemically stable than carbides which results in lower tendencies to stick to work piece material or wear by solution of the tool.
Besides Ti, the other metals of the groups VIa, Va and VIa, i.e., Zr, Hf, V, Nb, Ta, Cr, Mo and/or W, are normally used as hard constituent formers as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is generally &lt;2 .mu.m. As binder phase nowadays both cobalt and nickel are used. The amount of binder phase is generally 3-25% by weight. In addition, also other metals are used, for example aluminum, which sometimes are said to harden the binder phase and sometimes improve the wetting between hard constituents and binder phase, i.e., facilitate the sintering.
During sintering the relatively seen less stable hard constituents are dissolved in the binder phase and precipitate then as a rim on the more stable hard constituents. A very common structure in the alloys in question is therefore hard constituent grains with a core-rim structure. An early patent in this area is U.S. Pat. No. 3,971,656 which comprises Ti-and N-rich cores and rims rich in Mo, W and C. Through U.S. patent application Ser. No. 07/543,474 filed Jun. 26, 1990 and herein incorporated by reference, it is known that at least two different combinations of duplex core-rim-structures in well balanced proportions give optimal properties regarding wear resistance, toughness behavior and/or plastic deformation. Further examples of patents in this area are U.S. Pat. Nos. 4,904,445, 4,775,521, 4,957,548.
As a result of the dissolution of the hard constituents in the binder phase during sintering, the binder phase will contain a certain part of these in solid solution which affects the properties of the binder phase and thereby those of the whole alloy. The composition of the binder phase is determined by the starting raw materials as well as the way of manufacture, i.a., time and temperature during the sintering. It would be desirable to increase the alloying of group VI elements in order to obtain a more rigid alloy which gives improved resistance against mechanical stresses, i.e., a tougher behavior. However, such alloying has not heretofore been practically available.